1. Field of the Invention
The present invention relates to a power storage device and a method for manufacturing the power storage device.
2. Description of the Related Art
In recent years, power storage devices such as lithium-ion secondary batteries, lithium-ion capacitors, and air cells have been developed.
An electrode for a power storage device is manufactured by forming an active material over a surface of a current collector. As the active material, a material which is capable of occluding and releasing ions serving as carriers, such as carbon or silicon, is used. For example, silicon or phosphorus-doped silicon can occlude about four times as many ions serving as carriers as carbon. Thus, silicon or phosphorus-doped silicon has higher theoretical capacity than carbon and is advantageous in increasing the capacity of a power storage device (e.g., Patent Document 1).
However, an increase in the amount of occluded ions serving as carriers leads to a big change in the volume accompanied by occlusion and release of ions serving as carriers in the charge and discharge cycle and also leads to pulverization of silicon, which causes a problem in reliability.
Thus, silicon particle nuclei are coated with a carbon layer, whereby pulverization of silicon is prevented. The silicon particle nuclei contain silicon oxide as an impurity, and the silicon is coated with the carbon layer (see Patent Document 1).
On the other hand, in recent years, the use of graphene as a conductive electronic material in semiconductor devices has been studied. Graphene refers to a one-atom-thick sheet of carbon molecules having sp2 bonds.
Graphene is chemically stable and has favorable electric characteristics and thus has been expected to be applied to channel regions of transistors, vias, wirings, and the like included in semiconductor devices. In addition, an active electrode material is coated with graphene in order to increase the conductivity of an electrode material for a lithium-ion battery (see Patent Document 2).